FINE SEDIMENT TRANSPORT IN THE CONNECTICUT RIVER WATERSHED

An understanding of fine-sediment transport is necessary to describe the fate of particle-bound contaminants in rivers and the sea. We summarize results of an experimental study of silt and very-fine sand transport in channel flows, and then apply a key result of that study in an analysis of existing records of sediment load in selected rivers of the Connecticut River watershed. Specifically, we experimentally find that rates of fine-particle arrival at the bed become vanishingly small and rates of fine-particle re-entrainment from the bed potentially become very large when flow-induced lift just equals or exceeds the submerged weight of a suspended particle. This condition corresponds to flow speeds of about 50±10 cm s^-1 and, in flows in excess of this speed, fine particles can be expected to be in true suspension transport or wash load. In this context, an analysis of historical stream gauge and suspended sediment concentration data provide the following insights: 1) The relative difference between an observed suspended-sediment concentration and its predicted value using a standard rating curve is typically greater for discharges with flow speeds below ca. 50 cm s^-1 than for larger discharges and faster flows. In other words, the predictive performance of a sediment rating curve is notably better in the regime now known to be dominated strictly by suspension transport or wash load. 2) The fraction of days that newly-introduced silt and silt-bound contaminants may reach the bed, based on daily discharge values, are as follows: Ayers Brook (10² km² drainage area, undammed) ca. 50 percent; White River (10³ km², undammed) ca. 70 percent; Ompompanoosuc River (10²-10³ km², just below dam) ca. 65 percent; and Connecticut River at West Lebanon, NH (10⁴ km², just below dam) ca. 5 percent. These results tentatively suggest that river reaches of intermediate scale and discharge may be vulnerable to locally-introduced contaminant deposition to and sequestration in the channel bed.